1. Field of the Invention
This invention relates to a method of manufacturing semiconductor devices. More particularly, it concerns such a method by which contaminants, such as heavy metals, are removed from or prevented from inclusion in a semiconductor device when formed.
2. Description of the Related Art
In the manufacture of semiconductor devices, contaminants, particularly heavy metal contaminants such as Fe, Cu, and the like, are unintentionally mixed into the materials used for the manufacture and are precipitated in a substitutional site or an interstitial site of a semiconductor substrate, such a silicon substrate. As a result of the contaminants, generation recombination centers are formed, leakage currents at PN junctions are incurred, and the life of excess carriers is shortened. The electric characteristics of the device are also deteriorated by contaminants of the type referred to.
For example, in a MOS (metal oxide semiconductor) type memory device, when generated excess carriers, such as electrons or holes, are diffused in a silicon substrate, the stored charges in a charge stored memory cell are decreased in number. If the stored charges are decreased in number to total less than a critical charge, the state of the memory cell is inverted "1" to "0" and the storage data is lost.
Also, in a CCD (charge coupled device), excess carriers from a generation recombination center, are detected as a signal together with excess carriers caused by incident light. As a result, the excess carriers from the generation recombination center may provide an overly strong signal resulting in a white spot, for example, and the image quality is reduced.
Also, in a bipolar device, a generation recombination center becomes a cause of increased leakage currents at the PN junction. Excess charges generated in a base region are transferred to an output as an extraordinary signal, so low frequency noise is increased.
As mentioned above, the contamination by heavy metals causes the deterioration of the electric characteristics of the various semiconductor devices. Particularly, in the manufacture of an ULSI (Ultra Large Scale Integration) device, even if only a small quantity of contaminants are present in the device, the operating characteristics of the device are greatly deteriorated or changed Therefore, the contaminants become a main cause of decreased yield in the manufacture of semiconductor devices.
Two conventional techniques have been used to solve the problems associated with contaminants, i.e., sterilization and gettering.
Super cleaning technology has been developed to facilitate the manufacture of semiconductor devices by avoiding contaminants such as may be present in chemicals, e.g., hydrogen fluoride, nitric acid, hydrochloric acid, hydrogen peroxide, ammonium fluoride, and sulfuric acid, or deionized water, which are used in processes for manufacture of the device; or in the atmosphere such as dust in a clean room, and dust present on operators, and resist films, and dust represented by particles generated by several kinds of apparatus used for the manufacturing processes.
However, even if this technology is used to clean the environment for manufacturing an ULSI device, for example, as well as the material to be used, and to reduce the contaminants from the apparatuses used, it is difficult to completely control all of the process steps, e.g., several hundred steps, at the degree of cleanliness required for manufacture of the ULSI device. Thus, contamination in the course the process steps has continued to be a known statistical probability. That is, even if the presence of contaminants is controlled in all processes, the possibility for contamination is increased with increased numbers of the process steps so that generation of contamination cannot be avoided.
Examples of the so called gettering method of removing contaminants from active regions of a semiconductor device include, for example, phosphorus gettering, backside damage gettering and intrinsic gettering. In phosphorus getting, phosphorus is diffused from a back surface of a wafer during the final manufacturing process so that contaminants are moved from the wafer to an interface between the wafer substrate and the diffused phosphorus layer, and the contaminants are thus removed from active regions of a device. The phosphorus gettering is carried out, for example, by using POCl.sub.3 as a source gas and by exposing the wafer to an oxidized atmosphere at a temperature of between 900.degree. C. and 1000.degree. C.
In backside damage gettering, a mechanical strain is first intentionally imposed on the back surface of a wafer. Thereafter, during a first step for oxidizing in a ULSI manufacturing process, for example, oxidation induced stacking faults are caused at a nucleus caused by the mechanical strain. Contaminants like heavy metals are collected at the nucleus.
The mechanical strain is imposed, for example, by blowing fine SiO.sub.2 particles toward the back surface of the wafer. Because the oxidation induced stacking faults are grown fastest in an oxidization process at a temperature of 1100.degree. C., this gettering is especially effective in a high temperature process.
In intrinsic gettering, after a precipitate nucleus of oxygen is formed at a low temperature thermal process, such as at 650.degree. C. to 750.degree. C., oxygen is precipitated at a high temperature, such as 1000.degree. C. to 1100.degree. C. and heavy metals are taken into the oxygen. Also, to prevent generating precipitates at a surface of an active device region, a high temperature (e.g., 1200.degree. C.) thermal process is carried out in many cases before the low temperature of thermal process. The low temperature thermal process is usually carried out during a step for manufacturing a wafer, and the high temperature thermal process is carried out during a step for manufacturing an ULSI.
However, these gettering methods have problems. That is, a gettering method carried out during a process step for manufacturing a semiconductor wafer, such as damage gettering or intrinsic gettering referred to above, create a problem of increasing costs to form the wafer. Also, even in phosphorus gettering, as a gettering process is added, costs are likely to be increased.
Moreover, a high temperature thermal process, such as gettering, might create a problem. That is, because an ULSI is manufactured at microminiaturization scale, the distance between each device becomes extremely short. As a result, formation of a PN junction by ions, such as phosphorus, arsenic, or boron, and a partial doping by the ions should be carried out at a low temperature of less than 900.degree. C., for example, 800.degree. to 850.degree. C. However, as described above, in damage gettering, a high temperatures of more than 1000.degree. C. are necessary for the growth of oxidation-induced stacking faults. In intrinsic gettering, a temperature of more than 900.degree. C. is required to precipitate oxygen. Also in phosphorus gettering, due to a dependency on the temperature of the diffusion coefficient of phosphorus, the diffusion of phosphorus cannot be fully effected at other than high temperatures.
Therefore, because of the microminiaturized ULSI manufacture, a gettering at a low temperature is needed. However, contaminants cannot be removed fully at such the low temperature using the current state of the gettering arts. Accordingly, a method of manufacturing semiconductor devices, which can fully remove contaminants at relatively low temperatures, is desired.